U.S. Pat. No. 3,778,826 which is assigned to the assignee of the present application, teaches the use of an asymmetrical magic tee in an anti-collision vehicular radar system to provide phase information that can be readily detected to determine whether there is an opening or closing between the vehicle and obstacle which may be another vehicle. Work has also been accomplished by James E. Stevens and Louis L. Nagy as set forth in a paper entitled "Diplex Doppler Radar For Automotive Obstacle Detection" presented at the IEEE Vehicular Technology Group Conference on Dec. 6th, 1972, in Dallas Texas. This report analyzes the diplex Doppler technique which requires the use of two slightly different RF frequencies which are similarly transmitted by the radar, reflected off the vehicle, received by the radar and each mixed with its corresponding transmitted signal. An interesting relationship is found to exist wherein there is a phase difference between the signals of different RF frequencies which is a function of the exact difference frequency and more importantly the range from the vehicle to the obstacle.
However, it is noted in this report that with systems that are existing of this type there are a number of problems that have not been solved. For example, the prior art systems do not provide for adequate target discrimination and may be responsive to false conditions such as the presence of a relatively large overhead or roadside sign. In addition to discriminating between targets of different size, these prior art systems are also susceptible to detecting obstacles out of the path of the vehicle.
Accordingly, one object of the present invention is to provide an improved anti-collision vehicular radar system that overcomes the heretofore mentioned problems associated with the prior art systems. The system of the present invention is responsive primarily only to obstacles in the path of the vehicle and is not responsive to roadside obstacles including even relatively large roadside or overhead signs.
Another object of the present invention is to provide an improved anti-collision radar system that uses a diplex Doppler detection technique and that also measures vehicle speed relative to the ground which is useable in the system as a gain control.
A further object of the present invention is to provide for the system of this invention improved brake control means for pulsating the brakes in one of two different modes.
Still a further object of the present invention is to provide means for inhibiting brake application unless the detected vehicle speed is above a predetermined speed.